แปลงวันที่จาก ค.ศ. เป็น พ.ศ.ได้ แต่พอจะแปลงกลับเป็น ค.ศ. ไม่ได้

<?
	function DateThai($datetime)
	{
		$strYear = date("Y",strtotime($datetime))+543;
		$strMonth= date("n",strtotime($datetime));
		$strDay= date("j",strtotime($datetime));
		$strHour= date("H",strtotime($datetime));
		$strMinute= date("i",strtotime($datetime));
		$strSeconds= date("s",strtotime($datetime));
		$strMonthCut = Array("","01","02","03","04","05","06","07","08","09","10","11","12");
		$strMonthThai=$strMonthCut[$strMonth];
		return "$strDay/$strMonthThai/$strYear $strHour:$strMinute:$strSeconds";
	}
	
	function DateEng($datetime)
	{
		$strYear = date("Y",strtotime($datetime))-543;
		$strMonth= date("n",strtotime($datetime));
		$strDay= date("j",strtotime($datetime));
		$strHour= date("H",strtotime($datetime));
		$strMinute= date("i",strtotime($datetime));
		$strSeconds= date("s",strtotime($datetime));
		$strMonthCut = Array("","01","02","03","04","05","06","07","08","09","10","11","12");
		$strMonthThai=$strMonthCut[$strMonth];
		return "$strYear-$strMonthThai-$strDay $strHour:$strMinute:$strSeconds";
	}

	echo $datetime_now = date("Y-m-d H:i:s");
	echo "<br>";
	echo $datethai = DateThai($datetime_now);
	echo "<br>";
	echo DateEng($datethai);	
?>